just n idea...
suppose we had a thin multilayer-cap of comparable high capacitance with the mechanical length of an ic and a width of an ic. The cap would be connectible on both sides along the full length of it. We now could stuck this cap directly under the ic and get the shortest possible connection. Ic manufacturers could even manufacture and supplie a ready made "sandwich" like this.
What do you thinck? Would possible resulting strayfields pose problems that could not be tackled?

I could imagine a rectangular hole or recess in the board the cap goes in, just within the boundaries of the SMT pin pads, and the chip put on top of that. This would minimize stray fields and everything. I wouldn't be surprised to find something like this being done (and someone having one or more patents related to it despite it being obvious to me, though that's OT for this thread).

Quote:

Originally Posted by AndrewT

There was a post a few years back showing this.
It linked to a pdf where the decoupling was explained and further it showed that the route of the decoupling leads across the IC pins should follow the internal current route.

The book "High Speed Signal Design" shows this kind of thing, that a (sufficiently short) pulse along a trace will have its return current travel back right under it along the ground plane, as if the return current were only traveling through a shadow trace under the signal trace. I just found it, pp 189-190, "High-Speed current follows the path of least inductance." Thus it's good to keep conductors of both the "forward" and "return" currents close together, reducing the loop area as much as possible. As explained earlier in the book, putting a conductor closer to the groundplane also reduces generated EMI.

Quote:

Originally Posted by marce

You can get those sort of decouplers for PTH DIP components, but when you move to SMD, that where the problems arise.

I recall DIP sockets with a regular 0.1uF leaded capacitor mounted diagonally between the corner pins (7 and 14, or 8 and 16). That's about as short as you can get without mounting the cap on top and soldering the leads to the chip pins.

I recall DIP sockets with a regular 0.1uF leaded capacitor mounted diagonally between the corner pins (7 and 14, or 8 and 16). That's about as short as you can get without mounting the cap on top and soldering the leads to the chip pins.

Have a look at some of the stuff that is happening with PCB's these days, lots of fun stuff, you can even embedd 0201, 1005 chip components into the final build.Welcome

The best way for SMD decoupling with std circuit boards is to via the power pins to the planes and to via the capacitor pins to the plane, no routes between the caps and the power pins. This gives the lowest impedance, of course you require planes, we often have up to 16, with multiple grounds, though HDI makes life a lot easier and cuts down on the number of planes required. We then have to decied which power plane pairs hould be nearest the chips (ie top layer of PCB) and which should be near the bottom, made more fun when you have double sided placement...

Oh and of course with standards such as 74 logic, there was always an exception, the 74LS76

Wnet to an engineering day that Wurth ran recently, some of the stuff is getting, was amazed at some of the dev work they are playing with. One of the coolest that will be with us soon, is milling a cavity in the PCB and wire bonding the actual chips into the PCB, then sealing it in with the next layers. The prolifaration of mobile devices and the demand for more computing power, less energy used size and cost is driving the technology.
Flexi rigid are my favorites at the moment, a digital/analogue system could be built where the various parts could be seperated for noise, and with the flexi limbs I2S or similar (instead of SPDIF) could be used for the digital, without the signal integrity problems of going off board, and it could all be folded up neatly in a box. Done somthing similar, but its covered by NDA's etc. A similar idea is discussed here:Printed Circuit Design & Fab Magazine Online